Water-entrained-polyimide chemical compositions for use in high-performance composite fabrication

ABSTRACT

Water-entrained compositions comprising colloidal or suspensoidal solutions comprising polyimide pre-polymers/oligomers are described. These compositions are obtained in water by initial dispersion of the resin constituents in water to from colloids or suspensoids. The water-entrained polyimide compositions can be applied to numerous surfaces or more beneficially used for composite fabrication. The coated surfaces or polyimide-pre-polymer impregnated reinforcing materials are subsequently cured and are ideal for providing thermal protection.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to water-entrained-polyimidechemical compositions comprising stable aqueous-based colloidalsolutions for use in high-performance composite fabrication and methodsof making the same.

It is well known in the art that polyimides exhibit properties desirablefor high-temperature applications such as those demanded in theaerospace industry. Existing materials are primarily based onPolymerization of Monomeric Reactants (PMR) chemistry and are with a fewexceptions, only suitable for prepreg. Additionally, these materials areplagued with various deficiencies. For example, PMR-15, which isdescribed in U.S. Pat. No. 3,745,149, contains a known carcinogen4,4-methylene dianiline (MDA), is prone to microcracking, difficult tohandle during processing and has a short shelf life. Many PMR materialscannot be used for thick composites due to unwanted reaction byproducts.

Past methods, based primarily on PMR chemistry, of synthesizingpolyimide oligomers require the use of either dimethylformamide (DMF) oran alcohol-based solvent. The use of DMF leads to processingdifficulties due to the need for removing the DMF prior to the finalcure. The difficulty in removing DMF prompted the utilization ofalcohol-based systems for synthesizing polyimide oligomers (e.g. RP-46,PMR-II-50, PETI-330 and AFR-PE-4). However, the use of alcohol-basedsolvents in the synthesis of polyimides also has shortcomings. Inparticular, dissolution of the reactants in alcohol-based systems oftenleads to unwanted side reactions between the alcohol and the monomersand/or oligomers used in the reaction. Specifically, dissolution inalcohol, according to PMR chemistry, forms half esters that interferewith the formation of polyimides. These unwanted byproducts have anadverse impact on the properties of the finished products.

Accordingly, there continues to be a need for an aqueous-based systemfor polyimide oligomers suitable for high-performance composites thatmitigate health and toxicity problems, are easily processed and exhibitimproved thermo-oxidative stability. In particular, the aerospaceindustry presently has a need for composites and laminates that can besafely produced at a low cost and provide long-term thermal protectionfor temperatures up to 700° F.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention satisfy at least some of theaforementioned needs by providing an aqueous-based system for polyimideoligomers. Water-entrained compositions comprising colloidal solutionscomprising polyimide pre-polymers/oligomers are described. Thewater-entrained polyimide compositions are obtained by eitherco-reacting a polyamine and a polyanhydride with a specific endcappingmonomer or by directly reacting an amine-functional endcap monomer witha suitable chemical backbone. The water-entrained polyimide compositionscan be applied to numerous surfaces and used for composite fabrication.The coated surfaces or polyimide pre-polymer-impregnated reinforcingmaterials are subsequently cured and are ideal for providing thermalprotection.

Embodiments of the present invention comprise an aqueous-based methodfor preparing polyimide oligomers as colloids or suspensoids; whereinthe use of hazardous solvents are eliminated and undesirable sidereactions are avoided. Accordingly, embodiments of the present inventionprovide resin solutions suitable for preparing high-temperaturepolyimides suitable for composite fabrication.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter, inwhich some, but not all embodiments of the inventions are shown. Indeed,these inventions may be embodied in many different forms and should notbe construed as limited to the embodiments set forth herein; rather,these embodiments are provided so that this disclosure will satisfyapplicable legal requirements.

As previously discussed, polymeric resins for high temperaturecomposites have historically utilized solvents such as dimethylformamide(DMF) or alcohol-based systems for synthesizing polyimide oligomers.Embodiments of the present invention may be formed in accordance withany of the traditional approaches, namely PMR chemistry, except that anaqueous-based system is utilized. The use of DMF and other similarsolvents creates processing difficulties. On the other hand, dissolutionof reactants in alcohol-based systems often leads to unwanted sidereactions between the alcohol and the monomers and/or oligomers used inthe reaction. Specifically, dissolution in alcohol, according to PMRchemistry, forms half esters that interfere with the formation ofpolyimides. The problem of unwanted side reaction byproducts due to thedissolution of the reactants in alcohol is illustrated below.

In view of the inadequacies associated with the traditional approaches,embodiments of the present invention can be formed in an aqueous-basedsystem. In particular, embodiments of the present invention are formedby performing the polymerization reaction in water. As illustratedbelow, this approach eliminates the production of unwanted byproducts.

If desired, subsequent addition of alcohol to the aqueous-based systemdoes not have the same detrimental effect as when the monomers and/oroligomers are directly dissolved in alcohol because the side reaction ofthe anhydride with alcohol to form the diester will not occur at themoderate temperatures used for preparing coated fabrics or performssuitable for manufacturing composite parts. Thus, the addition ofalcohol to the aqueous-based system may be added to improve flow,wet-out, and increase tack (e.g. as an entraining material) withoutcausing unwanted reaction byproducts provided that the alcohol isremoved prior to processing at temperatures that will induceimidization. Accordingly, resins for use in high temperature polymericcomposites can easily be produced by using the aqueous-based system forpolyimide oligomers described above.

In one aspect, embodiments of the present invention relate towater-entrained compositions comprising colloidal or suspensoidalsolutions comprising polyimide oligomers/pre-polymers for use inhigh-performance composites and reinforced laminates of improved thermalstability and methods of making the same.

In one such embodiment, a water-entrained polyimide comprises thepolymerization reaction product of a polyfunctional amine (e.g. anaromatic diamine), polyfunctional anhydride (e.g. an aromaticdianhydride), and at least one mono-anhydride endcap; wherein thereaction is performed in an aqueous-based system.

Various polyfunctional amines such as diamines, triamines and tetraminesmay be used. In various embodiments, however, diamines are preferred.Polyfunctional amines suitable for specific embodiments include but arenot limited to, for example, the following compounds:

-   3-methoxyhexamethylene diamine;-   2,5-dimethylhexamethylene diamine;-   2,5-dimethylheptamethylene diamine;-   5-methylnonamethylene diamine;-   1,4-diamino-cyclohexane;-   1,2-diamino-octodecane;-   2,5-diamino-oxadiazole;-   2,2-bis(4-aminophenyl)hexafluoro propane;-   N-(3-aminophenyl)-4-aminobenzamide;-   metaphenylene diamine;-   para-phenylene diamine;-   4,4′-diamino-diphenyl propane;-   4,4′-diamino-diphenyl methane;-   benzidine;-   4,4′-diamino-diphenyl sulfide;-   4,4′-diamino-diphenyl sulfone;-   3,3′-diamino-diphenyl sulfone;-   4,4′-diamino-diphenyl ether;-   2,6-diamino-pyridine;-   bis-(4-amino-phenyl)diethyl silane;-   bis(4-amino-phenyl)diphenyl silane;-   3,3′-dichloro-benzidine;-   bis-(4-amino-phenyl)phenyl phosphide oxide;-   bis-(4-amino-phenyl)-N-phenylamine;-   bis-(4-amino-phenyl)-N-methyl-amine;-   1,5-diamino-naphthalene;-   3,3′-dimethyl-4,4′-diamino-biphenyl;-   3,4′-dimethyl-3′,4-diamino-biphenyl;-   3,3′-dimethoxy benzidine;-   2,4-bis(beta-amino-t-butyl) toluene;-   para-bis-(2-methyl-4-amino-pentyl)benzene;-   para-bis-(1,1-dimethyl-5-amino-pentyl)benzene;-   m-xylylene diamine;-   3-methylheptamethylene diamine;-   4,4-dimethyl-heptamethylene diamine;-   2,11-diamino-dodecane;-   1,2-bis-(3-aminopropoxy)ethane;-   2,2-dimethyl propylene diamine;-   1,3-diamino adamantane;-   3,3′-diamino-1,1′-diadamantane;-   3,3′-diaminomethyl-1,1′-diadamantane;-   bis(para-amino-cyclohexyl)methane;-   hexamethylene diamine;-   heptamethylene diamine;-   octamethylene diamine;-   nonamethylene diamine; and-   decamethylene diamine.

Polyfunctional anhydrides suitable for specific embodiments include butare not limited to, for example, the following compounds:

-   bis(3,4-dicarboxyphenyl)methane dianhydride;-   bis(3,4-dicarboxyphenyl) sulfone dianhydride;-   benzene-1,2,3,4-tetracarboxylic dianhydride;-   3,4,3′,4′-benzophenone tetracarboxylic dianhydride;-   pyromellitic dianhydride;-   2,3,6,7-naphthalene tetracarboxylic dianhydride;-   3,3′,4,4′-diphenyl tetracarboxylic dianhydride;-   1,2,5,6-naphthalene tetracarboxylic dianhydride;-   2,2′,3,3′-diphenyl tetracarboxylic dianhydride;-   2,2′-bis(3,4-dicarboxyphenyl) propane dianhydride;-   3,4,9,10-perylene tetracarboxylic dianhydride;-   bis(3,4-dicarboxyphenyl)ether dianhydride;-   naphthalene-1,2,4,5-tetracarboxylic dianhydride;-   naphtalene-1,4,5,8-tetracarboxylic dianhydride;-   decahydronaphthalene-1,4,5,8-tetracarboxylic dianhydride;-   phenanthrene-1,8,9,10-tetracarboxylic dianhydride;-   2,2-bis(2,3-dicarboxyphenyl) propane dianhydride;-   1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride;-   bis(2,3-dicarboxyphenyl)methane dianhydride;-   4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-1,2,5,6-tetracarboxylic    dianhydride;-   2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride;-   2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride; and-   2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride.

Mono-anhydride endcaps suitable for specific embodiments include but arenot limited to, for example, the following compounds:

In another embodiment, water-entrained polyimide compositions comprisethe polymerization reaction product of an amine-functional endcap and asuitable chemical backbone. In various embodiments, the chemicalbackbone may comprise an acid anhydride. Any endcap comprising aminefunctionality is contemplated, such as a nadic-based endcap.

Water-entrained polyimide compositions of embodiments of the presentinvention may be characterized as aqueous-based dispersions orcolloidal/suspensoidal solutions since the compositions comprisepolyimide pre-polymers which are not completely soluble in water.

The water-entrained polyimide compositions according to embodiments ofthe present invention may contain, if desired, various additives. Oneparticularly useful type of additive is stabilizing agents to preventflocculation of the polyimides. The stabilizers can also includeprotective colloids such as lignin derivatives, proteins, water-solublecellulose derivatives, alginic acid, long-chain alcohols, lecithin andsucrose-based stabilizing agents. It is imperative, however, to selectstabilizing agents such that they do not cause unwanted side reactionsand that are fugitive and do not remain in the final, processed part.

The water-entrained polyimide compositions in accordance withembodiments of the present invention can be formed by utilizing reactionschemes well known in the art for producing polyimides. Unlike priorapproaches, however, the preparation of materials suitable forfabricating polyimide composite materials is performed in anaqueous-based system instead of an organic, especially alcohol, solvent.In particular, the imidization reaction is carried-out after removal ofall water.

In one embodiment, water-entrained polyimide pre-polymers/oligomers areobtained by first removing the entrained water and then co-reactingpolyamines, polyanhydrides and a specific mono-anhydride inapproximately stoichiometric or predetermined amounts to form poly-amideacids which are subsequently heated to temperatures up to about 260° C.to form polyimide pre-polymers/oligomers in an aqueous-based system. Thepolyimide pre-polymers obtained from the polyamide-acids may becharacterized as chain-extended polyimides of relatively low molecularweight which contain an aliphatic and/or aromatic backbone with aspecified endcapping or terminal group. A particular endcapping groupshould be capable of becoming chemically reactive by the application ofelevated temperatures to form substantially cured polyimide resins. Itis believed that the terminal groups react with the application of heat,thereby causing the low-molecular-weight polyimide pre-polymers to addessentially end to end to form macromolecules having average molecularweights of at least 10,000.

The polyamide-acids/polyimide precursors may be obtained by conventionaltechniques, for example, by reacting a chemical equivalent of apolyamine, i.e. an aromatic diamine with an equivalent of anhydridecomprising a mixture of a dianhydride and a complimentary mono-anhydrideas is well known in the art (e.g.3,6-endomethylene-5-methyl-1,2,3,6-tetrahydrophthalic anhydride). Themono-anhydride is present in the anhydride mixture in an amountsufficient to endcap the polyimide pre-polymers. Thus, depending uponthe average molecular weight of the pre-polymers, the relative amount ofthe monoanhydride in the mixture will vary, e.g. from about 5.0 to 60mole percent. It is advantageous that the total chemical equivalents ofthe polyamine, i.e. diamine substantially equal the total equivalents ofthe dianhydride and monoanhydride so that a completely cyclizedpolyimide pre-polymer can be obtained in situ.

In various embodiments, for example, water-entrained polyimidepre-polymers are prepared as follows:

Polyimide compositions previously only available via PMR-based chemistry(alcohol-based systems) can now know be beneficially formed in anaqueous-based system according to embodiments of the present inventionto provide aqueous-based colloidal or suspensoidal compositions used incomposite fabrication. More specifically, examples of polyimidessuitable for composite fabrication that can be prepared in anaqueous-based system according to embodiments of the present inventioninclude those disclosed in U.S. Pat. No. 3,745,149 (Serafini et al.),U.S. Pat. No. 5,338,827 (Serafini et al.), U.S. Pat. No. 5,171,822(Pater), U.S. Pat. No. 5,081,198 (Pater et al.), U.S. Pat. No. 7,041,778(Curliss et al.), U.S. Pat. No. 6,958,192 (Hergenrother et al.), U.S.Pat. No. 5,412,066 (Hergenrother et al.), and U.S. Pat. No. 6,124,035(Hergenrother et al.).

U.S. Pat. No. 3,745,149 discloses the preparation of polyimides, namelyPMR-15, via a solution of monomeric reactants which react in situ toform double-endcap intermediates. PMR-15-type polyimides can be formedaccording to embodiments of the present invention by forming mixtures of(a) an dialkyl ester of an aromatic tetracarboxylic acid, (b) anaromatic diamine, and (c) an monoalkyl ester of a dicarboxylic acid (asan endcap) in a molar ratio of n:(n+1):2. The monomers are mixed in anaqueous-based system, introduced into a fibrous fabric or perform,laid-up into a desired configuration, have the water removed, and thenreacted at elevated temperatures to form, in situ, imide oligomershaving endcaps at both ends, and cured at high temperatures to yieldmacromolecular polyimides. For example, PMR-15 can be formed in anaqueous-based system as follows:

Additionally, polyimide resins as described in U.S. Pat. No. 5,338,827can also be formed by preparing a mixture in an aqueous-based systemcomprising the following monomers: (a) a dialkyl, trialkyl ortetraalkylester of biphenyltetracarboxylic acid; (b) phenylenediamine;and (c) a divalent endcap compound characterized by (i) having at leastone unsaturated moiety, (ii) being capable of reacting with the aromaticdiamine or the ester to form an endcap radical that precludes furtherreaction of the aromatic diamine with the ester, and (iii) being capableof undergoing addition polymerization. The molar ratio of (a), (b), and(c) can be adjusted such that heating the mixture formslow-molecular-weight pre-polymers having at least one endcap radicalsuitable for chain extension and crosslinking to formhigh-molecular-weight and thermally stable polyimides. The chemicalstructure, terminal moieties, and formulated molecular weights of thepre-polymers depend on the molar ratio of the reactants as described inU.S. Pat. No. 5,338,827.

U.S. Pat. No. 5,171,822 describes a polymerization of monomer reactants(PMR) system having 3,4′-oxydianiline as the key monomer reactant. Onevariation of this system, LaRC-RP46, is prepared by reacting togethermonomethyl ester or 5-norbornene-2,3-dicarboxylic acid (NE),3,4′-oxydianiline (3,4′-ODA), and dimethyl ester of3,3′,4,4′-benzophenonetetracarboxylic acid (BTDE); this combination isthen treated with heat. Embodiments of the present invention provide anaqueous-based system for forming water-entrained compositions comprisingthese polyimides.

U.S. Pat. No. 5,081,198 discloses the following compounds that can beformed as water-entrained compositions according to embodiments of thepresent invention:

U.S. Pat. No. 7,041,778 discloses a polyimide resin consistingessentially of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA),3,4,3′,4′-biphenyltetracarboxylic dianhydride (BPDA), 2,2bis(3′,4′-dicarboxy phenyl) hexafluoro propane dianhydride (6FDA),2-(3,4-dicarboxyphenyl)-1-phenylacetylene anhydride (4-PEPA) and anaromatic diamine. Prior to embodiments of the present invention, suchresins were only formed by utilization of alcohol-based systems.However, embodiments of the present invention provide an aqueous-basedsystem for forming water-entrained compositions comprising thesepolyimides.

U.S. Pat. No. 6,958,192 discloses polyimides prepared from2,3,3′,4′-biphenyltetracarboxylic dianhydride and a wide variety ofaromatic diamines and polyimides endcapped with phthalic anhydride.Water-entrained polyimides formed 2,3,3′,4′-biphenyltetracarboxylicdianhydride and aromatic diamines can also be formed in accordance withembodiments of the present invention in an aqueous-based system.

U.S. Pat. No. 5,412,066 describes a series of phenylethylenyl-terminatedimide oligomers that can be thermally cured to resins by employing thefollowing compounds to endcap imide oligomers:

Prior to embodiments of the present invention, phenylethynyl-terminatedimide oligomers were prepared by using solvents such asN,N-dimethylacetamide (DMAc), N-methylpyrrolidinone (NMP),N,N-dimethylformamide (DMF) or m-cresol. However, according toembodiments of the present invention, water-entrainedphenylethynyl-terminated imide oligomers are prepared in anaqueous-based system.

U.S. Pat. No. 6,124,035 discloses phenylethynyl containing oligomersprepared from aromatic diamines containing phenylethynyl groups andvarious ratios of phthalic anhydride and 4-phenylethynlphthalicanhydride in glacial acetic acid to form a mixture of imide compounds.These materials can by synthesized as water-entrained compositionsaccording to embodiments of the aqueous-based system of the presentinvention by reacting aromatic diamines containing phenylethynyl groupswith various ratios of phthalic anhydride (PA) and 4-phenylethynylphthalic anhydride (PEPA) in water.

Additionally, scientists at NASA-Lewis have developed a polyimide resincalled AMB-21 which replaces MDA with2,2-bis(4-[4-aminopnenoxyl]phenyl)propane (BAPP), a non-toxic,non-carcinogenic monomer. AMB-21 has further benefit in that it may beformed into composite components by using resin transfer molding (RTM).According to embodiments of the present invention, AMB-21 can also beobtained in embodiments of the aqueous-based system for formingpolyimides suitable for high-temperature composite fabrication.

For the aforementioned and the like, each can be obtained as awater-entrained composition via an aqueous-based system, for example, byhydrolyzing the anhydrides, mixing the necessary reactants and anydesired excipients such as stabilizing agents to ensure that the mixtureis in suspension (e.g. the mixture does not separate into multiplephases) and performing the initial imidization in the aqueous-basedsystem to form a water-entrained polyimide pre-polymer composition.These water-entrained-polyimide pre-polymer compositions can beconsolidated by applying heat to drive-off excess water during initiallayout. Curing of the polyimide pre-polymers can then be performed byfurther processing in the appropriate temperature range, as is known inthe art.

It has been found that comparatively high-molecular-weight polyimideresins can be dispersed throughout reinforcing materials, e.g. glass- orcarbon-fibers, by polymerizing the polyimide pre-polymers which haveaverage molecular weights ranging from about 500 to 6,000 and preferablyfrom 500 to 3,000. The polyimide pre-polymers are polymerized, in situ,to obtain improved laminated structures at temperatures ranging fromabout 200 to 350° C. and preferably at temperatures ranging from about250 to 350° C.

In yet another aspect, embodiments of the present invention are directedto polyimide composites/laminates comprising polyimide resin formed viaan aqueous-based system. More specifically, embodiments of thisparticular aspect of the present invention relates toreinforced-polyimide structures, e.g. glass or carbon laminates obtainedby impregnating reinforcing materials with the precursor of a polyimidepre-polymer which in turn is capable of being polymerized, in situ, to ahigher-molecular-weight thermally stable polyimide resin.

In various embodiments, polyimide resins are obtained and dispersedthroughout the reinforcing materials by applying an aqueous-basedcomposition comprising precursors comprising polyamide-acid to thematerials followed by a heating process whereby the precursors areconverted to relatively low-molecular-weight polyimide pre-polymerswhich are highly stable at ambient temperatures.

If desired, the colloidal particles may be passed through a colloid milluntil the desired dimensions are obtained. Colloid mills are generallyuseful for milling, dispersing, homogenizing and breaking down ofagglomerates in the manufacture of food pastes, emulsions, coatings,ointments, creams, pulps, grease etc. The main function of the colloidmill is to ensure a breakdown of agglomerates or in the case ofemulsions to produce droplets of fine size around 1 micron. Typically,the material to be processed is fed by gravity to the hopper or pumpedso as to pass between the rotor and stator elements where it issubjected to high shearing and hydraulic forces. Material is dischargedthrough a hopper whereby it can be re-circulated for a second pass ifneeded.

These polyimide pre-polymers however, are capable of being converted attemperatures above approximately 200° C. to higher-molecular-weightpolyimide resins which bonds the reinforcing material to give athermally stable composite. The polyimide pre-polymers may have averagemolecular weights ranging from about 500 to 6,000 and more preferablyfrom 500 to 3,000 and are sufficiently stable at ambient temperatures toallow for handling and storage under conditions considered adverse tothe polyimide pre-polymers used heretofore.

The reinforced-polyimide laminated structures presently available, priorto embodiments of the present invention, are known primarily because oftheir outstanding physical and chemical properties and particularlybecause of their stability at elevated temperatures. Thus, because ofthese and other attractive characteristics, the reinforced-polyimidestructures, e.g. glass or carbon laminates, have found numerousapplications in areas where high-strength and high-temperature materialsare needed. However, while the presently available polyimides aredesirable they are nevertheless economically at a disadvantage becauseof the difficulties encountered in handling the pre-polymers and inprocessing laminates and composites.

In this aspect of the invention, reinforced materials according toembodiments of the present invention may be obtained by a curingmechanism wherein polymerization of the polyimide pre-polymers takesplace upon fabrication in situ by the mere application of heat. Thisparticular technique comprises impregnating the reinforcing materialswith an aqueous-based composition comprising polyimide precursorscomprising polyamide-acids, followed by a drying operation attemperatures ranging up to about 260° C. which not only removes excesswater, but also volatiles formed during the cyclization of theprecursors to the lower molecular weight polyimide pre-polymers. Thepre-polymer-impregnated materials are substantially stable at ambienttemperatures and therefore may be stored if desired without anyunnecessary precautions for later use. Upon processing in thetemperature range of 200° C. to 350° C., the impregnated materials formintegral structures due to the polymerization of the polyimidepre-polymers. The materials impregnated with the polyimide resins inaccordance with this invention are thermally stable and for that reasonmay be used for a variety of purposes including, for example,heat-stable laminates, ablative heat-shields, and various other purposesparticularly in the aerospace industry.

More specifically, embodiments of the present invention relate to aprocess for preparing reinforced resinous laminates, e.g. glass- orcarbon-fiber laminates of improved thermal stability which comprisesimpregnating reinforcing materials with an effective amount of anaqueous-based composition comprising either polyimide precursorscomprising polyamide-acids. After impregnating the reinforcingmaterials, the precursor are converted in situ to low-molecular-weightpolyimide pre-polymers by the application of heat which completes thecyclization reaction and removes the volatiles from the structure. Thelaminated structures containing the polyimide pre-polymers aresubsequently cured anaerobically by subjecting the impregnated materialsto temperatures ranging from about 200° C. to 350° C. at pressuresranging from about atmospheric to 1,000 psi.

In preparing the laminated materials or reinforced articles, forexample, the precursors (e.g. polyamide-acids) are prepared in anaqueous-based system wherein the precursors are present in amountsranging from about 10 to 65 percent and more preferably 25 to 50 percentby weight of the solvent. Following the application of the precursors tothe reinforcing materials, excess water may be removed at elevatedtemperatures to reduce the total reaction time and to obtainsubstantially dried materials. Subsequently, the impregnated materialsare subjected to elevated temperatures ranging up to about 260° C. untilcompletely cyclized, comparatively low-molecular-weight polyimidepre-polymers are obtained in situ.

The pre-polymer-impregnated materials are then subjected to still highertemperatures ranging up to about 350° C. under pressures ranging fromatmospheric to 1,000 psi. or more until completely cured resinousstructures of improved thermal stability are obtained. Under theseconditions of pressure and heat, the cyclized imide pre-polymers arecured with the evolution of a minimum amount of volatiles to ahigher-molecular-weight resin which bonds the reinforcing material toform the structure.

Alternative embodiments for preparing reinforced resinous laminates,e.g. glass- or carbon-fiber laminates of improved thermal stabilitycomprise performing the initial imidization reaction in an aqueous-basedsystem to form a water-entrained composition comprising polyimidepre-polymers or oligomers, impregnating reinforcing materials with thewater-entrained composition, consolidating the composite by drying attemperatures ranging up to about 260° C. to remove excess water, andanaerobically curing the impregnated materials at a temperature range ofabout 200° C. to 350° C. at pressures ranging from about atmospheric to1,000 psi. to form integral structures due to the polymerization of thepolyimide pre-polymers/oligomers.

The reinforcing materials may be selected from a variety of knownorganic or inorganic powders or fibers including, for example, carbon,powdered metals, silicates, asbestos, synthetic fibers, natural fibers,metal filaments, metal oxide powders, and particularly glass or carbonfibers, e.g. glass mats, etc.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. A method of forming a pre-polymer-impregnatedpreform, comprising: (i) impregnating reinforcing materials with anaqueous-based composition to form a pre-impregnated preform, saidaqueous-based composition comprising (a) polyamide acids or (b)monomeric reactants including at least one polyamine, at least onepolyanhydride, and at least one endcap monomer; and (ii) drying thepre-impregnated preform to remove excess water; (iii) heating thepre-impregnated preform, subsequent to step (ii), to form apre-polymer-impregnated preform comprising polyimide pre-polymersimpregnated therein.
 2. The method according to claim 1, wherein theaqueous-based composition comprises a colloid or suspensoid.
 3. Themethod according to claim 1, wherein the aqueous-based compositioncomprises a dialkyl ester of an aromatic tetracarboxylic acid, anaromatic diamine, and an monoalkyl ester of a dicarboxylic acid.
 4. Themethod according to claim 1, wherein the aqueous-based compositioncomprises a dialkyl, trialkyl or tetraalkylester ofbiphenyltetracarboxylic acid; phenylenediamine; and a divalent end capcompound characterized by (i) having at least one unsaturated moiety,(ii) being capable of reacting with the aromatic diamine or the ester toform an endcap radical that precludes further reaction of the aromaticdiamine with the ester, and (iii) being capable of undergoing additionpolymerization.
 5. The method according to claim 1, wherein theaqueous-based composition comprises 5-norbornene-2,3-dicarboxylic acid;3,4′-oxydianiline; and a dimethyl ester of3,3′,4,4′-benzophenonetetracarboxylic acid.
 6. The method according toclaim 1, wherein the aqueous-based composition comprises3,3′,4,4′-benzophenonetetracarboxylic dianhydride;3,4,3′,4′-biphenyltetracarboxylic dianhydride; 2,2 bis(3′,4′-dicarboxyphenyl)hexafluoro propane dianhydride;2-(3,4-dicarboxyphenyl)-1-phenylacetylene anhydride and an aromaticdiamine.
 7. The method according to claim 1, wherein the aqueous-basedcomposition comprises 2,3,3′,4′-biphenyltetracarboxylic dianhydride;aromatic diamines; and phthalic anhydride endcaps.
 8. The methodaccording to claim 1, wherein the polyimide pre-polymers are endcappedwith at least one of the compounds selected from the group consistingof:


9. The method according to claim 1, wherein the at least one polyaminecomprises 2,2-bis(4-[4-aminopnenoxyl]phenyl)propane.